1. Field of Invention
The present invention relates to an electronic circuit, an electronic circuit driving method, an electro-optical device, a method of driving an electro-optical device, and an electronic device.
2. Description of Related Art
In recent years, electro-optical devices using organic EL elements as current-driven elements have been developed. Since a backlight is not required because organic EL elements are self-luminous elements, it is expected that electro-optical devices having display quality superior to that of other electro-optical devices in power consumption, the viewing angle, contrast, and the like, can be realized.
Among those types of electro-optical device, there is an electro-optical device called an active-matrix type in which pixel circuits for controlling the organic EL elements are arranged in a matrix on the display panel section thereof. The pixel circuits of the active-matrix-type electro-optical device have therein transistors for controlling the organic EL element. When a data signal for causing the display panel section to form a display is supplied from a data-line driving circuit to each pixel circuit, each pixel circuit controls the conductive state of the transistor in accordance with the data signal in order to control the organic EL element.
FIG. 10 is a circuit diagram showing an example of a conventional pixel circuit. A pixel circuit 80 is a pixel circuit of a voltage program method in which the data signal is a voltage signal. The pixel circuit 80 is formed of first and second transistors 81 and 82, a capacitor 83, and an organic EL element 84. The first transistor 81 is a p-channel FET, and the second transistor 82 is an n-channel FET.
The first transistor 81 is a transistor for controlling a driving current Id supplied to the organic EL element 84. The source of the first transistor 81 is connected to a driving power-supply section 85 having a driving voltage Vdd. The drain of the first transistor 81 is connected to the organic EL element 84. The gate of the first transistor 81 is connected to the drain of the second transistor 82. The magnitude of the driving voltage Vdd is set in advance in accordance with the range of the luminance gradation of the organic EL element 84.
The second transistor 82 functions as a switching transistor. The source of the second transistor 82 is connected to a data line U. The data line U is connected to the data-line driving circuit for supplying a data voltage Vd, which is the data signal. The gate of the second transistor 82 is connected to a scanning line S. The on/off state of the second transistor 82 is controlled in accordance with a scanning signal supplied from a scanning-line driving circuit via the scanning line S.
The capacitor 83 is connected between the gate and the source of the first transistor 81. The capacitor 83 is electrically connected to the data line U via the second transistor 82. In the capacitor 83, as a result of the second transistor 82 being turned on, an amount of electrical charge corresponding to the data voltage Vd is charged via the data line U.
In the pixel circuit 80 configured in this manner, first, a scanning signal for turning on the second transistor 82 in a predetermined data writing period is supplied to the gate of the second transistor 82 via the scanning line S from the scanning-line driving circuit. At that time, the second transistor 82 is turned on, and an amount of electrical charge corresponding to the data voltage Vd is charged in the capacitor 83 within the data writing period via the data line U. Then, after the data writing period ends, a scanning signal for turning off the second transistor 82 within a predetermined light-emitting period is supplied from the scanning-line driving circuit via the scanning line S to the gate of the second transistor 82. Then, the second transistor 82 is turned off, and the conductive state of the first transistor 81 is controlled on the basis of the charged voltage Vo corresponding to the amount of electrical charge stored in the capacitor 83 of the first transistor 81. Then, in the first transistor 81, a driving current Id corresponding to the charged voltage Vo is generated, and the driving current Id is supplied to the organic EL element 84. As a result, the luminance gradation of the organic EL element 84 is controlled in accordance with the driving current Id.
At this time, the first transistor 81 is set so as to operate in the saturated area. Therefore, the driving current Id of the first transistor 81 in the saturated area is expressed by the following equation:Id=(½)βo(Vo−Vth)2 where βo is the gain coefficient of the first transistor. When the carrier mobility of the first transistor is denoted as μ, the gate capacitance as A, the channel width as W, and the channel length as L, the gain coefficient βo is a constant expressed as βo=(βAW/L). Vth is the threshold voltage of the first transistor.
That is, the driving current Id is not directly related to the driving voltage Vdd, but is determined by the charged voltage Vo.
The power consumption Po of the organic EL element 84 is given on the basis of the following equation:
                              P          ⁢                                          ⁢          o                =                ⁢                  I          ⁢                                          ⁢                      d            ·            V                    ⁢                                          ⁢          d          ⁢                                          ⁢          d                                        =                ⁢                              (                          1              /              2                        )                    ⁢                                          ⁢                                                    βo                ⁡                                  (                                                            V                      ⁢                                                                                          ⁢                      o                                        -                                          V                      ⁢                                                                                          ⁢                      t                      ⁢                                                                                          ⁢                      h                                                        )                                            2                        ·            V                    ⁢                                          ⁢          d          ⁢                                          ⁢          d                    
Therefore, the power consumption Po is determined by the charged voltage Vo stored in the capacitor 83 and the driving voltage Vdd.